Catalytic converter
]] A catalytic converter (colloquially, "cat" or "catcon") is a device used to convert toxic exhaust emissions from an internal combustion engine into non-toxic substances. Inside a catalytic converter, a catalyst stimulates a chemical reaction in which noxious byproducts of combustion are converted to less toxic substances by dint of catalysed chemical reactions. The specific reactions vary with the type of catalyst installed. Most present-day vehicles that run on gasoline are fitted with a "three way" converter, so named because it converts the three main pollutants in automobile exhaust: an oxidising reaction converts carbon monoxide(CO) and unburned hydrocarbons(HC), and a reduction reaction converts oxides of nitrogen (NOx) to produce carbon dioxide(CO2), nitrogen(N2), and water(H2O).Catalytic Converters. International Platinum Group Metals Association. Retrieved January 10, 2011. The first widespread introduction of catalytic converters was in the United States market, where 1975 model year automobiles were so equipped to comply with tightening U.S. Environmental Protection Agency regulations on automobile exhaust emissions. The catalytic converters fitted were two-way models, combining carbon monoxide(CO) and unburned hydrocarbons(HC) to produce carbon dioxide(CO2) and water(H2O). Two-way catalytic converters of this type are now considered obsolete except on lean burn engines. Since most vehicles at the time used carburetors that provided a relatively rich air-fuel ratio, oxygen (O2) levels in the exhaust stream were in general insufficient for the catalytic reaction to occur. Therefore, most such engines were also equipped with secondary air injection systems to induct air into the exhaust stream to allow the catalyst to function. Catalytic converters are still most commonly used on automobile exhaust systems, but are also used on generator sets, forklifts, mining equipment, trucks, buses, locomotives, airplanes and other engine fitted devices. This is usually in response to government regulation, either through direct environmental regulation or through Health and Safety regulations. History The catalytic converter was invented by Eugene Houdry, a French mechanical engineer and expert in catalytic oil refining who lived in the U.S. around 1950. When the results of early studies of smog in Los Angeles were published, Houdry became concerned about the role of automobile exhaust in air pollution and founded a special company, Oxy-Catalyst, to develop catalytic converters for gasoline engines. He was awarded United States Patent 2742437 for his work. Widespread adoption of catalytic converters didn't occur until more stringent emission control regulations forced the removal of anti-knock agent tetraethyl lead from most gasoline, because lead was a 'catalyst poison' and would inactivate the converter by forming a coating on the catalyst's surface, effectively disabling it.Staff writer (undated). "Eugene Houdry". Chemical Heritage Foundation. Retrieved January 7, 2011. Catalytic converters were further developed by a series of engineers including John J. Mooney and Carl D. Keith at the Engelhard Corporation, "Carl D. Keith, a Father of the Catalytic Converter, Dies at 88". The New York Times. November 15, 2008. creating the first production catalytic converter in 1973. Staff writer (undated). "Engelhard Corporation". referenceforbusiness.com. Retrieved January 7, 2011. Construction The catalytic converter consists of several components: # The catalyst core, or substrate. For automotive catalytic converters, the core is usually a ceramic monolith with a honeycomb structure. Metallic foil monoliths made of FeCrAl are used in some applications. This is partially a cost issue. Ceramic cores are inexpensive when manufactured in large quantities. Metallic cores are less expensive to build in small production runs. Either material is designed to provide a high surface area to support the catalyst washcoat, and therefore is often called a "catalyst support". The cordierite ceramic substrate used in most catalytic converters was invented by Rodney Bagley, Irwin Lachman and Ronald Lewis at Corning Glass, for which they were inducted into the National Inventors Hall of Fame in 2002. # The washcoat. A washcoat is a carrier for the catalytic materials and is used to disperse the materials over a high surface area. Aluminum oxide, Titanium dioxide, Silicon dioxide, or a mixture of silica and alumina can be used. The catalytic materials are suspended in the washcoat prior to applying to the core. Washcoat materials are selected to form a rough, irregular surface, which greatly increases the surface area compared to the smooth surface of the bare substrate. This maximizes the catalytically active surface available to react with the engine exhaust. # The catalyst itself is most often a precious metal. Platinum is the most active catalyst and is widely used, but is not suitable for all applications because of unwanted additional reactions and high cost. Palladium and rhodium are two other precious metals used. Rhodium is used as a reduction catalyst, palladium is used as an oxidation catalysts, and platinum is used both for reduction and oxidation. Cerium, iron, manganese and nickel are also used, although each has its own limitations. Nickel is not legal for use in the European Union (because of its reaction with carbon monoxide into nickel tetracarbonyl). Copper can be used everywhere except North America, where its use is illegal because of the formation of dioxin. Types Two-way A two-way (or "oxidation") catalytic converter has two simultaneous tasks: # Oxidation of carbon monoxide to carbon dioxide: 2CO + O2 → 2CO2 # Oxidation of hydrocarbons (unburnt and partially-burnt fuel) to carbon dioxide and water: CxH2x+2 + (3x+1)/2 O2 → xCO2 + (x+1) H2O (a combustion reaction) This type of catalytic converter is widely used on diesel engines to reduce hydrocarbon and carbon monoxide emissions. They were also used on gasoline engines in American- and Canadian-market automobiles until 1981. Because of their inability to control oxides of nitrogen, they were superseded by three-way converters. Three-way Since 1981, three-way (oxidation-reduction) catalytic converters have been used in vehicle emission control systems in the United States and Canada; many other countries have also adopted stringent vehicle emission regulations that in effect require three-way converters on gasoline-powered vehicles. A three-way catalytic converter has three simultaneous tasks: # Reduction of nitrogen oxides to nitrogen and oxygen: 2NOx → xO2 + N2 # Oxidation of carbon monoxide to carbon dioxide: 2CO + O2 → 2CO2 # Oxidation of unburnt hydrocarbons (HC) to carbon dioxide and water: CxH2x+2 + (3x+1)/2O2 → xCO2 + (x+1)H2O These three reactions occur most efficiently when the catalytic converter receives exhaust from an engine running slightly above the stoichiometric point. This point is between 14.6 and 14.8 parts air to 1 part fuel, by weight, for gasoline. The ratio for Autogas (or liquefied petroleum gas (LPG)), natural gas and ethanol fuels is each slightly different, requiring modified fuel system settings when using those fuels. In general, engines fitted with 3-way catalytic converters are equipped with a computerized closed-loop feedback fuel injection system using one or more oxygen sensors, though early in the deployment of three-way converters, carburetors equipped for feedback mixture control were used. Three-way catalysts are effective when the engine is operated within a narrow band of air-fuel ratios near stoichiometry, such that the exhaust gas oscillates between rich (excess fuel) and lean (excess oxygen) conditions. However, conversion efficiency falls very rapidly when the engine is operated outside of that band of air-fuel ratios. Under lean engine operation, there is excess oxygen and the reduction of NOx is not favored. Under rich conditions, the excess fuel consumes all of the available oxygen prior to the catalyst, thus only stored oxygen is available for the oxidation function. Closed-loop control systems are necessary because of the conflicting requirements for effective NOx reduction and HC oxidation. The control system must prevent the NOx reduction catalyst from becoming fully oxidized, yet replenish the oxygen storage material to maintain its function as an oxidation catalyst. Oxygen storage Three-way catalytic converters can store oxygen from the exhaust gas stream, usually when the air-fuel ratio goes lean. When insufficient oxygen is available from the exhaust stream, the stored oxygen is released and consumed (see cerium(IV) oxide). A lack of sufficient oxygen occurs either when oxygen derived from NOx reduction is unavailable or when certain maneuvers such as hard acceleration enrich the mixture beyond the ability of the converter to supply oxygen. Unwanted reactions Unwanted reactions can occur in the three-way catalyst, such as the formation of odoriferous hydrogen sulfide and ammonia. Formation of each can be limited by modifications to the washcoat and precious metals used. It is difficult to eliminate these byproducts entirely. Sulfur-free or low-sulfur fuels eliminate or reduce hydrogen sulfide. For example, when control of hydrogen-sulfide emissions is desired, nickel or manganese is added to the washcoat. Both substances act to block the adsorption of sulfur by the washcoat. Hydrogen sulfide is formed when the washcoat has adsorbed sulfur during a low-temperature part of the operating cycle, which is then released during the high-temperature part of the cycle and the sulfur combines with HC. For diesel engines For compression-ignition (i.e., diesel engines), the most-commonly-used catalytic converter is the Diesel Oxidation Catalyst (DOC). This catalyst uses O2 (oxygen) in the exhaust gas stream to convert CO (carbon monoxide) to CO2 (carbon dioxide) and HC (hydrocarbons) to H2O (water) and CO2. These converters often operate at 90 percent efficiency, virtually eliminating diesel odor and helping to reduce visible particulates (soot). These catalyst are not active for NOx reduction because any reductant present would react first with the high concentration of O2 in diesel exhaust gas. Reduction in NOx emissions from compression-ignition engine has previously been addressed by the addition of exhaust gas to incoming air charge, known as exhaust gas recirculation (EGR). In 2010, most light-duty diesel manufactures in the U.S. added catalytic systems to their vehicles to meet new federal emissions requirements. There are two techniques that have been developed for the catalytic reduction of NOx emissions under lean exhaust condition - selective catalytic reduction (SCR) and the lean NOx trap or NOx adsorber. Instead of precious metal-containing NOx adsorbers, most manufacturers selected base-metal SCR systems that use a reagent such as ammonia to reduce the NOx into nitrogen. Ammonia is supplied to the catalyst system by the injection of urea into the exhaust, which then undergoes thermal decomposition and hydrolysis into ammonia. One trademark product of urea solution, also referred to as Diesel Emission Fluid (DEF), is AdBlue. Diesel exhaust contains relatively high levels of particulate matter (soot), consisting in large part of elemental carbon. Catalytic converters cannot clean up elemental carbon, though they do remove up to 90 percent of the soluble organic fraction , so particulates are cleaned up by a soot trap or diesel particulate filter (DPF). A DPF consists of a Cordierite or Silicon Carbide substrate with a geometry that forces the exhaust flow through the substrate walls, leaving behind trapped soot particles. As the amount of soot trapped on the DPF increases, so does the back pressure in the exhaust system. Periodic regenerations (high temperature excursions) are required to initiate combustion of the trapped soot and thereby reducing the exhaust back pressure. The amount of soot loaded on the DPF prior to regeneration may also be limited to prevent extreme exotherms from damaging the trap during regeneration. In the U.S., all on-road light, medium and heavy-duty vehicles powered by diesel and built after January 1, 2007, must meet diesel particulate emission limits that means they effectively have to be equipped with a 2-Way catalytic converter and a diesel particulate filter. Note that this applies only to the diesel engine used in the vehicle. As long as the engine was manufactured before January 1, 2007, the vehicle is not required to have the DPF system. This led to an inventory runup by engine manufacturers in late 2006 so they could continue selling pre-DPF vehicles well into 2007. Lean Burn Spark Ignition Engines For Lean Burn spark-ignition engines, an oxidation catalyst is used in the same manner as in a diesel engine. Emissions from Lean Burn Spark Ignition Engines are very similar to emissions from a Diesel Compression Ignition engine. Installation Many vehicles have a close-coupled catalysts located near the engine's exhaust manifold. This unit heats up quickly due to its proximity to the engine, and reduces cold-engine emissions by burning off hydrocarbons from the extra-rich mixture used to start a cold engine. In the past, some three-way catalytic converter systems used an air-injection tube between the first (NOx reduction) and second (HC and CO oxidation) stages of the converter. This tube was part of a secondary air injection system. The injected air provided oxygen for the oxidation reactions. An upstream air injection point was also sometimes present to provide oxygen during engine warmup, which caused unburned fuel to ignite in the exhaust tract before reaching the catalytic converter. This cleaned up the exhaust and reduced the engine runtime needed for the catalytic converter to reach its "light-off" or operating temperature. Most modern catalytic converter systems do not have air injection systems. Instead, they provide a constantly varying air-fuel mixture that quickly and continually cycles between lean and rich exhaust. Oxygen sensors are used to monitor the exhaust oxygen content before and after the catalytic converter and this information is used by the Electronic control unit to adjust the fuel injection so as to prevent the first (NOx reduction) catalyst from becoming oxygen-loaded while ensuring the second (HC and CO oxidization) catalyst is sufficiently oxygen-saturated. The reduction and oxidation catalysts are typically contained in a common housing, however in some instances they may be housed separately. Damage Poisoning Catalyst poisoning occurs when the catalytic converter is exposed to exhaust containing substances that coat the working surfaces, encapsulating the catalyst so that it cannot contact and treat the exhaust. The most-notable contaminant is lead, so vehicles equipped with catalytic converters can be run only on unleaded gasoline. Other common catalyst poisons include fuel sulfur, manganese (originating primarily from the gasoline additive MMT), and silicone, which can enter the exhaust stream if the engine has a leak, allowing coolant into the combustion chamber. Phosphorus is another catalyst contaminant. Although phosphorus is no longer used in gasoline, it (and zinc, another low-level catalyst contaminant) was until recently widely used in engine oil antiwear additives such as zinc dithiophosphate (ZDDP). Beginning in 2006, a rapid phaseout of ZDDP in engine oils began. Depending on the contaminant, catalyst poisoning can sometimes be reversed by running the engine under a very heavy load for an extended period of time. The increased exhaust temperature can sometimes liquefy or sublime the contaminant, removing it from the catalytic surface. However, removal of lead deposits in this manner is usually not possible because of lead's high boiling point. Meltdown Any condition that causes abnormally high levels of unburned hydrocarbons — raw or partially burnt fuel — to reach the converter will tend to significantly elevate its temperature, bringing the risk of a meltdown of the substrate and resultant catalytic deactivation and severe exhaust restriction. Vehicles equipped with OBD-II diagnostic systems are designed to alert the driver to a misfire condition by means of flashing the "check engine" light on the dashboard. Regulations Emissions regulations vary considerably from jurisdiction to jurisdiction. The earliest on-road regulations which forced the use of Catalytic converters were the California For Non-Road regulations California led the way with its 2001 Large Spark Ignition Engine Regulation. This was followed by the United States Environmental Protection Agency 50 State Program for Non-Road spark-ignition engines of over output built after January 1, 2004, are equipped with three-way catalytic converters. In Japan, a similar set of regulations came into effect January 1, 2007. The European Union has regulations beginning with Euro 1 regulations in 1992 and becoming progressively more stringent in subsequent years. Most automobile spark-ignition engines in North America have been fitted with catalytic converters since the mid-1970s, and the technology used in non-automotive applications is generally based on automotive technology. Regulations for diesel engines are similarly varied, with some jurisdictions focusing on NOx (nitric oxide and nitrogen dioxide) emissions and others focusing on particulate (soot) emissions. This regulatory diversity is challenging for manufacturers of engines, as it may not be economical to design an engine to meet two sets of regulations. Regulations of fuel quality vary across jurisdictions. In North America, Europe, Japan and Hong Kong, gasoline and diesel fuel are highly regulated, and compressed natural gas and LPG (Autogas) are being reviewed for regulation. In most of Asia and Africa, the regulations are often lax — in some places sulfur content of the fuel can reach 20,000 parts per million (2%). Any sulfur in the fuel can be oxidized to SO2 (sulfur dioxide) or even SO3 (sulfur trioxide) in the combustion chamber. If sulfur passes over a catalyst, it may be further oxidized in the catalyst, i.e., SO2 may be further oxidized to SO3. Sulfur oxides are precursors to sulfuric acid, a major component of acid rain. While it is possible to add substances such as vanadium to the catalyst washcoat to combat sulfur-oxide formation, such addition will reduce the effectiveness of the catalyst. The most effective solution is to further refine fuel at the refinery to produce ultra-low sulfur diesel. Regulations in Japan, Europe and North America tightly restrict the amount of sulfur permitted in motor fuels. However, the expense of producing such clean fuel may make it impractical for use in developing countries. As a result, cities in these countries with high levels of vehicular traffic suffer from acid rain, which damages stone and woodwork of buildings, poisons humans and other animals, and damages local ecosystems. Negative aspects Some early converter designs greatly restricted the flow of exhaust, which negatively affected vehicle performance, driveability, and fuel economy. Because they were used with carburetors incapable of precise fuel-air mixture control, they could overheat and set fire to flammable materials under the car. |date=June 14, 1976|title=Catalytic Converter Still Controversial after Two Years of Use|author=Ullman, Owen}} Removing a modern catalytic converter in new condition will not increase vehicle performance without retuning,Catalytic Converter Removal – Beat the Law – Import Tuner Magazine. Importtuner.com (2007-02-26). Retrieved on 2011-01-09. but their removal or "gutting" continues. The exhaust section where the converter was may be replaced with a welded-in section of straight pipe, or a flanged section of "test pipe" legal for off-road use that can then be replaced with a similarly fitted converter-choked section for legal on-road use, or emissions testing. In the U.S. and many other jurisdictions, it is illegal to remove or disable a catalytic converter for any reason other than its immediate replacement . It is a violation of Section 203(a)(3)(A) of the 1990 Clean Air Act for a vehicle owner to remove a converter from their own vehicle. Section 203(a)(3)(B) makes it illegal for any person to sell or to install any part where a principle effect would be to bypass, defeat, or render inoperative any device or element of design of a vehicles emission control system. Vehicles without functioning catalytic converters generally fail emission inspections. The automotive aftermarket supplies high-flow converters for vehicles with upgraded engines, or whose owners prefer an exhaust system with larger-than-stock capacity. Warm-up period Most of the pollution put out by a car occurs during the first five minutes before the catalytic converter has warmed up sufficiently.Catalytic converters, nsls.bnl.gov In 1999, BMW introduced the Electric Catalytic Convert, or "E-CAT", in their flagship E38 750iL sedan. Coils inside the catalytic converter assemblies are heated electrically just after engine start, bringing the catalyst up to operating temperature much faster than traditional catalytic converters can, providing cleaner cold starts and low emission vehicle (LEV) compliance. Environmental impact Catalytic converters have proven to be reliable and effective in reducing noxious tailpipe emissions. However, they may have some adverse environmental impacts in use: *The requirement for an internal combustion engine equipped with a three-way catalyst to run at the stoichiometric point means it is less efficient than if it were operated lean. Thus, there is an increases the amount of fossil fuel consumed and the carbon-dioxide emissions from the vehicle. However, NOx control on lean-burn engines is problematic and requires special lean NOx catalysts to meet U.S. emissions regulations. *Although catalytic converters are effective at removing hydrocarbons and other harmful emissions, they do not solve the fundamental problem created by burning a fossil fuel. In addition to water, the main combustion product in exhaust gas leaving the engine — through a catalytic converter or not — is carbon dioxide (CO2). Carbon dioxide produced from fossil fuels is one of the greenhouse gases indicated by the Intergovernmental Panel on Climate Change (IPCC) to be a "most likely" cause of global warming. Additionally, the U.S. EPA has stated catalytic converters are a significant and growing cause of global warming, because of their release of nitrous oxide (N2O), a greenhouse gas over three hundred times more potent than carbon dioxide. *Catalytic converter production requires palladium or platinum; part of the world supply of these precious metals is produced near Norilsk, Russia, where the industry (among others) has caused Norilsk to be added to Time magazine's list of most-polluted places. Theft Because of the external location and the use of valuable precious metals including platinum, palladium, and rhodium, converters are a target for thieves. The problem is especially common among late-model Toyota trucks and SUVs, because of their high ground clearance and easily removed bolt-on catalytic converters. Welded-in converters are also at risk of theft from SUVs and trucks, as they can be easily removed."Catalytic Converter Theft".Murr, Andrew (January 9, 2008). "An Exhausting New Crime — What Thieves Are Stealing from Today's Cars". Newsweek. Retrieved January 7, 2011. Theft removal of the converter can often inadvertently damage the car's wiring or fuel line resulting in dangerous consequences. Rises in metal costs in the U.S. during recent years have led to a large increase in theft incidents of the converter,Johnson, Alex (February 12, 2008). "Stolen in 60 Seconds: The Treasure in Your Car — As Precious Metals Prices Soar, Catalytic Converters Are Targets for Thieves". MSNBC. Retrieved January 7, 2011. which can then cost as much as $1,000 to replace. Diagnostics Various jurisdictions now legislate on-board diagnostics to monitor the function and condition of the emissions-control system, including the catalytic converter. On-board diagnostic systems take several forms. Temperature sensors Temperature sensors are used for two purposes. The first is as a warning system, typically on two-way catalytic converters such as are still sometimes used on LPG forklifts. The function of the sensor is to warn of catalytic converter temperature above the safe limit of . More-recent catalytic-converter designs are not as susceptible to temperature damage and can withstand sustained temperatures of . Temperature sensors are also used to monitor catalyst functioning — usually two sensors will be fitted, with one before the catalyst and one after to monitor the temperature rise over the catalytic-converter core. For every 1% of CO in the exhaust gas stream, the exhaust gas temperature will rise by 100 °C. Oxygen sensors The oxygen sensor is the basis of the closed-loop control system on a spark-ignited rich-burn engine; however, it is also used for diagnostics. In vehicles with OBD II, a second oxygen sensor is fitted after the catalytic converter to monitor the O2 levels. The on-board computer makes comparisons between the readings of the two sensors. If both sensors show the same output, the computer recognizes that the catalytic converter either is not functioning or has been removed, and will operate a "check engine" light and retard engine performance. Simple "oxygen sensor simulators" have been developed to circumvent this problem by simulating the change across the catalytic converter with plans and pre-assembled devices available on the Internet. Although these are not legal for on-road use, they have been used with mixed results. Similar devices apply an offset to the sensor signals, allowing the engine to run a more fuel-economical lean burn that may, however, damage the engine or the catalytic converter. NOx sensors NOx sensors are extremely expensive and are in general used only when a compression-ignition engine is fitted with a selective catalytic-reduction (SCR) converter, or a NOx absorber catalyst in a feedback system. When fitted to an SCR system, there may be one or two sensors. When one sensor is fitted it will be pre-catalyst; when two are fitted, the second one will be post-catalyst. They are used for the same reasons and in the same manner as an oxygen sensor — the only difference is the substance being monitored. See also *Automobile emissions control *Catalysis *Cerium(III) oxide *Exhaust system *NOx adsorbers *Roadway air dispersion modeling References Patents * Keith, C. D., et al., – – "Apparatus for purifying exhaust gases of an internal combustion engine" – April 29, 1969 * Lachman, I. M. et al., – – ''"Anisotropic Cordierite Monolith" (Ceramic substrate) – November 5, 1973 * Charles H. Bailey, – – "Combination muffler and catalytic converter having low backpressure" – June 13, 1978 * Charles H. Bailey, – – '"Caseless monolithic catalytic converter" – February 10, 1981 * Srinivasan Gopalakrishnan – – ''"Process And Synthesizer For Molecular Engineering Of Materials" – March 13, 2002 External links *Howstuffworks: "How Catalytic Converters Work" *High Temperature Insulation Wool — Automotive Applications Category:Air pollution control systems Category:American inventions Category:Automotive technologies Category:Catalysis Category:Exhaust systems Category:NOx control